Cassini Data Reveals Unexpected Features In Titan’s Icy Outer Shell

August 28, 2013

This image of the surface of Saturn's moon Titan was obtained by the European Space Agency's Huygens Probe on Jan. 14, 2005, after it was delivered to Titan by NASA's Cassini spacecraft. Credit: ESA/NASA/JPL/University of Arizona

Cassini, a joint NASA/ESA/ASI orbiter, arrived at Saturn a little over nine years and has been observing the ringed planet and its moons ever since. The latest data from the spacecraft shows that the planet’s largest moon, Titan, has some unexpected features.

A new study published in the journal Nature reveals that Titan’s rigid outer ice shell has some features quite different from any on Earth.

“Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain. On Titan, when you fly over a mountain the gravity gets lower. That’s a very odd observation,” Francis Nimmo at the University of California, Santa Cruz, said in a statement.

Nimmo and colleagues developed a model to help explain such an observation. With this model, each bump in the topography on the surface of Titan is offset by a deeper “root” big enough to overwhelm the gravitational effect of the bump on the surface. These roots are like icebergs extending below the ice shell into an ocean underneath.

“Because ice is lower density than water, you get less gravity when you have a big chunk of ice there than when you have water,” Nimmo explained.

The buoyancy of an Iceberg floating on water balances out its weight, but on Titan the roots extending below the ice sheet are so much bigger than the bumps on the surface that their buoyancy is causing them to push up against the ice sheet.

“It’s like a big beach ball under the ice sheet pushing up on it, and the only way to keep it submerged is if the ice sheet is strong,” said Douglas Hemingway, a doctoral candidate in planetary geophysics at UCSC and lead author of the paper. “If large roots are the reason for the negative correlation, it means that Titan’s ice shell must have a very thick rigid layer.”

Given the new model, scientists can assume Titan’s ice shell would need a rigid layer at least 24 miles thick. They also found that thousands of feet of surface erosion and deposition would be required in order to account for the observed imbalance between the large roots and small surface topography.

Titan’s shell isn’t being recycled by plate tectonics like Earth’s, but instead Nimmo says that it is “just sitting there, and weather and erosion are acting on it, moving stuff around and redepositing sediments.”

“It may be like the surface of Earth would be if you turned plate tectonics off,” the researcher said.

The scientists are not quite sure what could have kick-started Titan’s topographical features with their deep roots. However, Hemingway pointed out that Titan’s eccentric orbit around Saturn generates tides that flex the moon’s surface and create tidal heating. This could cause variations to develop in the thickness of the ice shell.

Image Caption (below): This diagram of a cross-section through Titan’s ice shell shows features that may explain the gravity anomaly: a low-density ice lens created by regional basal freezing; a rigid ice shell that resists upward deflection; and surface weathering that keeps topography small. Credit: D. Hemingway